Is there a Snap option in the Graph Editor?
Ok so Mr Zak suggested me the best method to do it:


Select the vertice #1 which is the Y position reference.
ctrl G to make the cursor move to the vertice position.
Select the 2D Cursor as the Pivot Point.
Select all the vertices you want to snap to the Y position.
Scale down to zero with S Y 0


and a quite similar way:


Select the vertice #1, which is your Y reference, ctrl G to move your green cursor up to this vertice.
Select the vertices you want to align on the same Y position
shift S > Cursor Value, and again shift S > Flatten Handles, if necessary.